Results from laboratory have shown previously that mouse strains of the Tlad genotype express thymus leukemia antigens (TLA) of two different size classes (bearing heavy chains of 45,000 and 50,000 daltons, each with a typical beta 2m light chain). Our purpose is to determine the chemical basis for this structural heterogeneity. We are inducing thymic leukemias in b10,M, and A.CA mice by fractionated X-irradiation in order to provide a source of TLA plus cells which will incorporate radioisotopic precursors into TLA. Similar studies are planned with the Tlae strain, C3H.NB. Another group of experiments is directed at testing the hypothesis that TLA plus thymocytes differentiate into peripheral T cells. These studies make use of cell transfers between Tla congenic mouse strains. Reconstitution of lethally X-irradiated, thymectomized recipients will be achieved with immunospecifically selected TLA plus (cortical) or TLA minus (medullary) thymocytes and excess bone marrow cells. The transferred thymus cells will then be the limiting cell type in the recipient's antibody response to T cell dependent antigens.